A Serbian far-right party says its leader, Vojislav Seselj, has undergone surgery after defying orders to return to a United Nations war crimes tribunal that released him temporarily for health reasons.

The ultranationalist Serbian Radical Party says Seselj, who is accused of recruiting paramilitary forces during the 1990s wars in Croatia and Bosnia-Herzegovina, had the operation on May 7.

The party did not give further details, saying only that it was awaiting further information from Seselj's Serbian doctors.

The doctors have previously said that Seselj was suffering from colon cancer that has spread to his liver.

The U.N. war crimes tribunal at The Hague temporarily released Seselj in November on condition that he would return to the court if summoned.

In March, appeals judges ruled that Seselj must return to the tribunal. But Seselj has refused to do so.